<!DOCTYPE html>
<html lang="zh-CN" data-theme="light">
  <head>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width,initial-scale=1" />
    <meta name="generator" content="VuePress 2.0.0-beta.38" />
    <meta name="theme" content="VuePress Theme Hope" />
    <meta property="og:url" content="https://javaguide.cn/java/collection/java-collection-precautions-for-use.html"><meta property="og:site_name" content="JavaGuide"><meta property="og:title" content="Java集合使用注意事项总结"><meta property="og:type" content="article"><meta property="og:updated_time" content="2022-03-03T01:14:56.000Z"><meta property="og:locale" content="zh-CN"><meta property="article:tag" content="Java集合"><meta property="article:modified_time" content="2022-03-03T01:14:56.000Z"><script>var _hmt = _hmt || [];
      (function() {
        var hm = document.createElement("script");
        hm.src = "https://hm.baidu.com/hm.js?5dd2e8c97962d57b7b8fea1737c01743";
        var s = document.getElementsByTagName("script")[0]; 
        s.parentNode.insertBefore(hm, s);
      })();</script><link rel="stylesheet" href="//at.alicdn.com/t/font_2922463_99aa80ii7cf.css"><title>Java集合使用注意事项总结 | JavaGuide</title><meta name="description" content="Java学习&&面试指南">
    <style>
      :root {
        --bg-color: #fff;
      }

      html[data-theme="dark"] {
        --bg-color: #1d2025;
      }

      html,
      body {
        background-color: var(--bg-color);
      }
    </style>
    <script>
      const userMode = localStorage.getItem("vuepress-theme-hope-scheme");
      const systemDarkMode =
        window.matchMedia &&
        window.matchMedia("(prefers-color-scheme: dark)").matches;

      if (userMode === "dark" || (userMode !== "light" && systemDarkMode)) {
        document.querySelector("html").setAttribute("data-theme", "dark");
      }
    </script>
    <link rel="stylesheet" href="/assets/style.aa943f56.css">
    <link rel="modulepreload" href="/assets/app.93341f6d.js"><link rel="modulepreload" href="/assets/java-collection-precautions-for-use.html.9503e766.js"><link rel="modulepreload" href="/assets/java-collection-precautions-for-use.html.0900b814.js"><link rel="modulepreload" href="/assets/plugin-vue_export-helper.21dcd24c.js">
  </head>
  <body>
    <div id="app"><!--[--><!--[--><!--[--><span tabindex="-1"></span><a href="#main-content" class="skip-link sr-only">Skip to content</a><!--]--><div class="theme-container has-toc sidebar-open"><!--[--><header class="navbar"><button class="toggle-sidebar-button" title="Toggle Sidebar"><span class="icon"></span></button><a href="/" class="home-link"><img class="logo" src="/logo.png" alt="JavaGuide"><!----><span class="site-name hide-in-pad">JavaGuide</span><!--[--><!----><!--]--></a><nav class="nav-links" style=""><div class="nav-item hide-in-mobile"><a href="/home.html" class="nav-link" arialabel="面试指南"><i class="icon iconfont icon-java"></i>面试指南<!----></a></div><div class="nav-item hide-in-mobile"><a href="/zhuanlan/" class="nav-link" arialabel="优质专栏"><i class="icon iconfont icon-recommend"></i>优质专栏<!----></a></div><div class="nav-item hide-in-mobile"><a href="/open-source-project/" class="nav-link" arialabel="项目精选"><i class="icon iconfont icon-github"></i>项目精选<!----></a></div><div class="nav-item hide-in-mobile"><a href="/books/" class="nav-link" arialabel="书籍精选"><i class="icon iconfont icon-book"></i>书籍精选<!----></a></div><div class="nav-item hide-in-mobile"><a href="https://snailclimb.gitee.io/javaguide/#/" rel="noopener noreferrer" target="_blank" arialabel="旧版链接" class="nav-link"><i class="icon iconfont icon-java"></i>旧版链接<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span><!----></a></div><div class="nav-item hide-in-mobile"><a href="https://javaguide.cn/feed.json" rel="noopener noreferrer" target="_blank" arialabel="RSS订阅" class="nav-link"><i class="icon iconfont icon-rss"></i>RSS订阅<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span><!----></a></div><div class="nav-item hide-in-mobile"><a href="/about-the-author/" class="nav-link" arialabel="关于作者"><i class="icon iconfont icon-zuozhe"></i>关于作者<!----></a></div></nav><div class="nav-actions-wrapper"><!--[--><!----><!--]--><div class="nav-item"><!----></div><div class="nav-item"><a class="repo-link" href="https://github.com/Snailclimb/JavaGuide" target="_blank" rel="noopener noreferrer"><svg xmlns="http://www.w3.org/2000/svg" class="icon github-icon" viewbox="0 0 1024 1024" arialabelledby="github" style="width:1.25rem;height:1.25rem;vertical-align:middle;"><title id="github" lang="en">github icon</title><g fill="currentColor"><path d="M511.957 21.333C241.024 21.333 21.333 240.981 21.333 512c0 216.832 140.544 400.725 335.574 465.664 24.49 4.395 32.256-10.07 32.256-23.083 0-11.69.256-44.245 0-85.205-136.448 29.61-164.736-64.64-164.736-64.64-22.315-56.704-54.4-71.765-54.4-71.765-44.587-30.464 3.285-29.824 3.285-29.824 49.195 3.413 75.179 50.517 75.179 50.517 43.776 75.008 114.816 53.333 142.762 40.79 4.523-31.66 17.152-53.377 31.19-65.537-108.971-12.458-223.488-54.485-223.488-242.602 0-53.547 19.114-97.323 50.517-131.67-5.035-12.33-21.93-62.293 4.779-129.834 0 0 41.258-13.184 134.912 50.346a469.803 469.803 0 0 1 122.88-16.554c41.642.213 83.626 5.632 122.88 16.554 93.653-63.488 134.784-50.346 134.784-50.346 26.752 67.541 9.898 117.504 4.864 129.834 31.402 34.347 50.474 78.123 50.474 131.67 0 188.586-114.73 230.016-224.042 242.09 17.578 15.232 33.578 44.672 33.578 90.454v135.85c0 13.142 7.936 27.606 32.854 22.87C862.25 912.597 1002.667 728.747 1002.667 512c0-271.019-219.648-490.667-490.71-490.667z"></path></g></svg></a></div><div class="nav-item hide-in-mobile"><button id="appearance-switch"><svg xmlns="http://www.w3.org/2000/svg" class="icon auto-icon" viewbox="0 0 1024 1024" arialabelledby="auto" style="display:block;"><title id="auto" lang="en">auto icon</title><g fill="currentColor"><path d="M512 992C246.92 992 32 777.08 32 512S246.92 32 512 32s480 214.92 480 480-214.92 480-480 480zm0-840c-198.78 0-360 161.22-360 360 0 198.84 161.22 360 360 360s360-161.16 360-360c0-198.78-161.22-360-360-360zm0 660V212c165.72 0 300 134.34 300 300 0 165.72-134.28 300-300 300z"></path></g></svg><svg xmlns="http://www.w3.org/2000/svg" class="icon dark-icon" viewbox="0 0 1024 1024" arialabelledby="dark" style="display:none;"><title id="dark" lang="en">dark icon</title><g fill="currentColor"><path d="M524.8 938.667h-4.267a439.893 439.893 0 0 1-313.173-134.4 446.293 446.293 0 0 1-11.093-597.334A432.213 432.213 0 0 1 366.933 90.027a42.667 42.667 0 0 1 45.227 9.386 42.667 42.667 0 0 1 10.24 42.667 358.4 358.4 0 0 0 82.773 375.893 361.387 361.387 0 0 0 376.747 82.774 42.667 42.667 0 0 1 54.187 55.04 433.493 433.493 0 0 1-99.84 154.88 438.613 438.613 0 0 1-311.467 128z"></path></g></svg><svg xmlns="http://www.w3.org/2000/svg" class="icon light-icon" viewbox="0 0 1024 1024" arialabelledby="light" style="display:none;"><title id="light" lang="en">light icon</title><g fill="currentColor"><path d="M952 552h-80a40 40 0 0 1 0-80h80a40 40 0 0 1 0 80zM801.88 280.08a41 41 0 0 1-57.96-57.96l57.96-58a41.04 41.04 0 0 1 58 58l-58 57.96zM512 752a240 240 0 1 1 0-480 240 240 0 0 1 0 480zm0-560a40 40 0 0 1-40-40V72a40 40 0 0 1 80 0v80a40 40 0 0 1-40 40zm-289.88 88.08-58-57.96a41.04 41.04 0 0 1 58-58l57.96 58a41 41 0 0 1-57.96 57.96zM192 512a40 40 0 0 1-40 40H72a40 40 0 0 1 0-80h80a40 40 0 0 1 40 40zm30.12 231.92a41 41 0 0 1 57.96 57.96l-57.96 58a41.04 41.04 0 0 1-58-58l58-57.96zM512 832a40 40 0 0 1 40 40v80a40 40 0 0 1-80 0v-80a40 40 0 0 1 40-40zm289.88-88.08 58 57.96a41.04 41.04 0 0 1-58 58l-57.96-58a41 41 0 0 1 57.96-57.96z"></path></g></svg></button></div><form class="search-box" role="search"><input type="search" placeholder="搜索" autocomplete="off" spellcheck="false" value><!----></form><button class="toggle-navbar-button" aria-label="Toggle Navbar" aria-expanded="false" aria-controls="nav-screen"><span class="button-container"><span class="button-top"></span><span class="button-middle"></span><span class="button-bottom"></span></span></button><!--[--><!----><!--]--></div></header><!----><!--]--><!----><div class="toggle-sidebar-wrapper"><span class="arrow left"></span></div><aside class="sidebar"><!--[--><!----><!--]--><ul class="sidebar-links"><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><i class="icon iconfont icon-mianshi"></i><span class="title">面试准备</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable active"><i class="icon iconfont icon-java"></i><span class="title">Java</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><i class="icon iconfont icon-basic"></i><span class="title">基础</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable active"><i class="icon iconfont icon-container"></i><span class="title">容器</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><!--[--><a href="/java/collection/java-collection-questions-01.html" class="nav-link sidebar-link sidebar-page" arialabel="Java集合常见知识点&amp;面试题总结(上)"><!---->Java集合常见知识点&amp;面试题总结(上)<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/java/collection/java-collection-questions-02.html" class="nav-link sidebar-link sidebar-page" arialabel="Java集合常见知识点&amp;面试题总结(下)"><!---->Java集合常见知识点&amp;面试题总结(下)<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" arialabel="Java集合使用注意事项总结"><!---->Java集合使用注意事项总结<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html#集合判空" class="router-link-active router-link-exact-active nav-link sidebar-link heading" arialabel="集合判空"><!---->集合判空<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html#集合转-map" class="router-link-active router-link-exact-active nav-link sidebar-link heading" arialabel="集合转 Map"><!---->集合转 Map<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html#集合遍历" class="router-link-active router-link-exact-active nav-link sidebar-link heading" arialabel="集合遍历"><!---->集合遍历<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html#集合去重" class="router-link-active router-link-exact-active nav-link sidebar-link heading" arialabel="集合去重"><!---->集合去重<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html#集合转数组" class="router-link-active router-link-exact-active nav-link sidebar-link heading" arialabel="集合转数组"><!---->集合转数组<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html#数组转集合" class="router-link-active router-link-exact-active nav-link sidebar-link heading" arialabel="数组转集合"><!---->数组转集合<!----></a><ul class="sidebar-sub-headers"></ul></li></ul><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">源码分析</span><span class="arrow right"></span></button><!----></section><!--]--></li></ul></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><i class="icon iconfont icon-et-performance"></i><span class="title">并发编程</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><i class="icon iconfont icon-virtual_machine"></i><span class="title">JVM</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><i class="icon iconfont icon-features"></i><span class="title">新特性</span><span class="arrow right"></span></button><!----></section><!--]--></li></ul></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><i class="icon iconfont icon-computer"></i><span class="title">计算机基础</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><i class="icon iconfont icon-database"></i><span class="title">数据库</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><i class="icon iconfont icon-Tools"></i><span class="title">开发工具</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><i class="icon iconfont icon-xitongsheji"></i><span class="title">系统设计</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><i class="icon iconfont icon-distributed-network"></i><span class="title">分布式</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><i class="icon iconfont icon-et-performance"></i><span class="title">高性能</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><i class="icon iconfont icon-CalendarAvailability-1"></i><span class="title">高可用</span><span class="arrow right"></span></button><!----></section><!--]--></li></ul><!--[--><!----><!--]--></aside><!--[--><main class="page" id="main-content"><!----><nav class="breadcrumb disable"></nav><div class="page-title"><h1><!---->Java集合使用注意事项总结</h1><div class="article-info"><span class="author-info" arialabel="作者🖊" isoriginal="false" pageview="false" color="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon author-icon" viewbox="0 0 1024 1024" arialabelledby="author"><title id="author" lang="en">author icon</title><g fill="currentColor"><path d="M649.6 633.6c86.4-48 147.2-144 147.2-249.6 0-160-128-288-288-288s-288 128-288 288c0 108.8 57.6 201.6 147.2 249.6-121.6 48-214.4 153.6-240 288-3.2 9.6 0 19.2 6.4 25.6 3.2 9.6 12.8 12.8 22.4 12.8h704c9.6 0 19.2-3.2 25.6-12.8 6.4-6.4 9.6-16 6.4-25.6-25.6-134.4-121.6-240-243.2-288z"></path></g></svg><span><a class="author-item" href="https://javaguide.cn/" target="_blank" rel="noopener noreferrer">Guide</a></span><span property="author" content="Guide"></span></span><span class="category-info" arialabel="分类🌈" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon category-icon" viewbox="0 0 1024 1024" arialabelledby="category"><title id="category" lang="en">category icon</title><g fill="currentColor"><path d="M148.41 106.992h282.176c22.263 0 40.31 18.048 40.31 40.31V429.48c0 22.263-18.047 40.31-40.31 40.31H148.41c-22.263 0-40.311-18.047-40.311-40.31V147.302c0-22.263 18.048-40.31 40.311-40.31zM147.556 553.478H429.73c22.263 0 40.311 18.048 40.311 40.31v282.176c0 22.263-18.048 40.312-40.31 40.312H147.555c-22.263 0-40.311-18.049-40.311-40.312V593.79c0-22.263 18.048-40.311 40.31-40.311zM593.927 106.992h282.176c22.263 0 40.31 18.048 40.31 40.31V429.48c0 22.263-18.047 40.31-40.31 40.31H593.927c-22.263 0-40.311-18.047-40.311-40.31V147.302c0-22.263 18.048-40.31 40.31-40.31zM730.22 920.502H623.926c-40.925 0-74.22-33.388-74.22-74.425V623.992c0-41.038 33.387-74.424 74.425-74.424h222.085c41.038 0 74.424 33.226 74.424 74.067v114.233c0 10.244-8.304 18.548-18.547 18.548s-18.548-8.304-18.548-18.548V623.635c0-20.388-16.746-36.974-37.33-36.974H624.13c-20.585 0-37.331 16.747-37.331 37.33v222.086c0 20.585 16.654 37.331 37.126 37.331H730.22c10.243 0 18.547 8.304 18.547 18.547 0 10.244-8.304 18.547-18.547 18.547z"></path></g></svg><ul class="categories-wrapper"><li class="category clickable" role="navigation">Java</li><meta property="articleSection" content="Java"></ul></span><span arialabel="标签🏷" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon tag-icon" viewbox="0 0 1024 1024" arialabelledby="tag"><title id="tag" lang="en">tag icon</title><g fill="currentColor"><path d="M939.902 458.563L910.17 144.567c-1.507-16.272-14.465-29.13-30.737-30.737L565.438 84.098h-.402c-3.215 0-5.726 1.005-7.634 2.913l-470.39 470.39a10.004 10.004 0 000 14.164l365.423 365.424c1.909 1.908 4.42 2.913 7.132 2.913s5.223-1.005 7.132-2.913l470.39-470.39c2.01-2.11 3.014-5.023 2.813-8.036zm-240.067-72.121c-35.458 0-64.286-28.828-64.286-64.286s28.828-64.285 64.286-64.285 64.286 28.828 64.286 64.285-28.829 64.286-64.286 64.286z"></path></g></svg><ul class="tags-wrapper"><li class="tag clickable" role="navigation">Java集合</li></ul><meta property="keywords" content="Java集合"></span><span class="date-info" arialabel="写作日期📅" isoriginal="false" pageview="false" color="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon calendar-icon" viewbox="0 0 1024 1024" arialabelledby="calendar"><title id="calendar" lang="en">calendar icon</title><g fill="currentColor"><path d="M716.4 110.137c0-18.753-14.72-33.473-33.472-33.473-18.753 0-33.473 14.72-33.473 33.473v33.473h66.993v-33.473zm-334.87 0c0-18.753-14.72-33.473-33.473-33.473s-33.52 14.72-33.52 33.473v33.473h66.993v-33.473zm468.81 33.52H716.4v100.465c0 18.753-14.72 33.473-33.472 33.473a33.145 33.145 0 01-33.473-33.473V143.657H381.53v100.465c0 18.753-14.72 33.473-33.473 33.473a33.145 33.145 0 01-33.473-33.473V143.657H180.6A134.314 134.314 0 0046.66 277.595v535.756A134.314 134.314 0 00180.6 947.289h669.74a134.36 134.36 0 00133.94-133.938V277.595a134.314 134.314 0 00-133.94-133.938zm33.473 267.877H147.126a33.145 33.145 0 01-33.473-33.473c0-18.752 14.72-33.473 33.473-33.473h736.687c18.752 0 33.472 14.72 33.472 33.473a33.145 33.145 0 01-33.472 33.473z"></path></g></svg><span>2022年3月3日</span><meta property="datePublished" content="2022-03-03T01:14:56.000Z"></span><!----><span class="words-info" arialabel="字数🔠" isoriginal="false" pageview="false" color="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon word-icon" viewbox="0 0 1024 1024" arialabelledby="word"><title id="word" lang="en">word icon</title><g fill="currentColor"><path d="M518.217 432.64V73.143A73.143 73.143 0 01603.43 1.097a512 512 0 01419.474 419.474 73.143 73.143 0 01-72.046 85.212H591.36a73.143 73.143 0 01-73.143-73.143z"></path><path d="M493.714 566.857h340.297a73.143 73.143 0 0173.143 85.577A457.143 457.143 0 11371.566 117.76a73.143 73.143 0 0185.577 73.143v339.383a36.571 36.571 0 0036.571 36.571z"></path></g></svg><span>约 2501 字</span><meta property="wordCount" content="2501"></span></div><hr></div><div class="toc-place-holder"><aside id="toc-list"><div class="toc-header">此页内容</div><div class="toc-wrapper"><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html#集合判空" class="router-link-active router-link-exact-active toc-link level2">集合判空</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html#集合转-map" class="router-link-active router-link-exact-active toc-link level2">集合转 Map</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html#集合遍历" class="router-link-active router-link-exact-active toc-link level2">集合遍历</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html#集合去重" class="router-link-active router-link-exact-active toc-link level2">集合去重</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html#集合转数组" class="router-link-active router-link-exact-active toc-link level2">集合转数组</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/java/collection/java-collection-precautions-for-use.html#数组转集合" class="router-link-active router-link-exact-active toc-link level2">数组转集合</a></li><!----><!--]--></ul></div></aside></div><!----><div class="theme-hope-content"><!--[--><p>这篇文章我根据《阿里巴巴 Java 开发手册》总结了关于集合使用常见的注意事项以及其具体原理。</p><p>强烈建议小伙伴们多多阅读几遍，避免自己写代码的时候出现这些低级的问题。</p><h2 id="集合判空" tabindex="-1"><a class="header-anchor" href="#集合判空" aria-hidden="true">#</a> 集合判空</h2><p>《阿里巴巴 Java 开发手册》的描述如下：</p><blockquote><p><strong>判断所有集合内部的元素是否为空，使用 <code>isEmpty()</code> 方法，而不是 <code>size()==0</code> 的方式。</strong></p></blockquote><p>这是因为 <code>isEmpty()</code> 方法的可读性更好，并且时间复杂度为 O(1)。</p><p>绝大部分我们使用的集合的 <code>size()</code> 方法的时间复杂度也是 O(1)，不过，也有很多复杂度不是 O(1) 的，比如 <code>java.util.concurrent</code> 包下的某些集合（<code>ConcurrentLinkedQueue</code> 、<code>ConcurrentHashMap</code>...）。</p><p>下面是 <code>ConcurrentHashMap</code> 的 <code>size()</code> 方法和 <code>isEmpty()</code> 方法的源码。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">long</span> n <span class="token operator">=</span> <span class="token function">sumCount</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>n <span class="token operator">&lt;</span> <span class="token number">0L</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token number">0</span> <span class="token operator">:</span>
            <span class="token punctuation">(</span>n <span class="token operator">&gt;</span> <span class="token punctuation">(</span><span class="token keyword">long</span><span class="token punctuation">)</span><span class="token class-name">Integer</span><span class="token punctuation">.</span>MAX_VALUE<span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token class-name">Integer</span><span class="token punctuation">.</span>MAX_VALUE <span class="token operator">:</span>
            <span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span>n<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">final</span> <span class="token keyword">long</span> <span class="token function">sumCount</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">CounterCell</span><span class="token punctuation">[</span><span class="token punctuation">]</span> as <span class="token operator">=</span> counterCells<span class="token punctuation">;</span> <span class="token class-name">CounterCell</span> a<span class="token punctuation">;</span>
    <span class="token keyword">long</span> sum <span class="token operator">=</span> baseCount<span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>as <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> as<span class="token punctuation">.</span>length<span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>a <span class="token operator">=</span> as<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
                sum <span class="token operator">+=</span> a<span class="token punctuation">.</span>value<span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> sum<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">isEmpty</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">sumCount</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&lt;=</span> <span class="token number">0L</span><span class="token punctuation">;</span> <span class="token comment">// ignore transient negative values</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><h2 id="集合转-map" tabindex="-1"><a class="header-anchor" href="#集合转-map" aria-hidden="true">#</a> 集合转 Map</h2><p>《阿里巴巴 Java 开发手册》的描述如下：</p><blockquote><p><strong>在使用 <code>java.util.stream.Collectors</code> 类的 <code>toMap()</code> 方法转为 <code>Map</code> 集合时，一定要注意当 value 为 null 时会抛 NPE 异常。</strong></p></blockquote><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">class</span> <span class="token class-name">Person</span> <span class="token punctuation">{</span>
    <span class="token keyword">private</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span>
    <span class="token keyword">private</span> <span class="token class-name">String</span> phoneNumber<span class="token punctuation">;</span>
     <span class="token comment">// getters and setters</span>
<span class="token punctuation">}</span>

<span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Person</span><span class="token punctuation">&gt;</span></span> bookList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
bookList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">&quot;jack&quot;</span><span class="token punctuation">,</span><span class="token string">&quot;18163138123&quot;</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
bookList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token string">&quot;martin&quot;</span><span class="token punctuation">,</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 空指针异常</span>
bookList<span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span><span class="token class-name">Collectors</span><span class="token punctuation">.</span><span class="token function">toMap</span><span class="token punctuation">(</span><span class="token class-name">Person</span><span class="token operator">::</span><span class="token function">getName</span><span class="token punctuation">,</span> <span class="token class-name">Person</span><span class="token operator">::</span><span class="token function">getPhoneNumber</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>下面我们来解释一下原因。</p><p>首先，我们来看 <code>java.util.stream.Collectors</code> 类的 <code>toMap()</code> 方法 ，可以看到其内部调用了 <code>Map</code> 接口的 <code>merge()</code> 方法。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">,</span> <span class="token class-name">K</span><span class="token punctuation">,</span> <span class="token class-name">U</span><span class="token punctuation">,</span> <span class="token class-name">M</span> <span class="token keyword">extends</span> <span class="token class-name">Map</span><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span> <span class="token class-name">U</span><span class="token punctuation">&gt;</span><span class="token punctuation">&gt;</span></span>
<span class="token class-name">Collector</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">,</span> <span class="token operator">?</span><span class="token punctuation">,</span> <span class="token class-name">M</span><span class="token punctuation">&gt;</span></span> <span class="token function">toMap</span><span class="token punctuation">(</span><span class="token class-name">Function</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span> <span class="token keyword">super</span> <span class="token class-name">T</span><span class="token punctuation">,</span> <span class="token operator">?</span> <span class="token keyword">extends</span> <span class="token class-name">K</span><span class="token punctuation">&gt;</span></span> keyMapper<span class="token punctuation">,</span>
                            <span class="token class-name">Function</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span> <span class="token keyword">super</span> <span class="token class-name">T</span><span class="token punctuation">,</span> <span class="token operator">?</span> <span class="token keyword">extends</span> <span class="token class-name">U</span><span class="token punctuation">&gt;</span></span> valueMapper<span class="token punctuation">,</span>
                            <span class="token class-name">BinaryOperator</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">U</span><span class="token punctuation">&gt;</span></span> mergeFunction<span class="token punctuation">,</span>
                            <span class="token class-name">Supplier</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">M</span><span class="token punctuation">&gt;</span></span> mapSupplier<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">BiConsumer</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">M</span><span class="token punctuation">,</span> <span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> accumulator
            <span class="token operator">=</span> <span class="token punctuation">(</span>map<span class="token punctuation">,</span> element<span class="token punctuation">)</span> <span class="token operator">-&gt;</span> map<span class="token punctuation">.</span><span class="token function">merge</span><span class="token punctuation">(</span>keyMapper<span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>element<span class="token punctuation">)</span><span class="token punctuation">,</span>
                                          valueMapper<span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>element<span class="token punctuation">)</span><span class="token punctuation">,</span> mergeFunction<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">CollectorImpl</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>mapSupplier<span class="token punctuation">,</span> accumulator<span class="token punctuation">,</span> <span class="token function">mapMerger</span><span class="token punctuation">(</span>mergeFunction<span class="token punctuation">)</span><span class="token punctuation">,</span> CH_ID<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p><code>Map</code> 接口的 <code>merge()</code> 方法如下，这个方法是接口中的默认实现。</p><blockquote><p>如果你还不了解 Java 8 新特性的话，请看这篇文章：<a href="https://mp.weixin.qq.com/s/ojyl7B6PiHaTWADqmUq2rw" target="_blank" rel="noopener noreferrer">《Java8 新特性总结》<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a> 。</p></blockquote><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">default</span> <span class="token class-name">V</span> <span class="token function">merge</span><span class="token punctuation">(</span><span class="token class-name">K</span> key<span class="token punctuation">,</span> <span class="token class-name">V</span> value<span class="token punctuation">,</span>
        <span class="token class-name">BiFunction</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span> <span class="token keyword">super</span> <span class="token class-name">V</span><span class="token punctuation">,</span> <span class="token operator">?</span> <span class="token keyword">super</span> <span class="token class-name">V</span><span class="token punctuation">,</span> <span class="token operator">?</span> <span class="token keyword">extends</span> <span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> remappingFunction<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">Objects</span><span class="token punctuation">.</span><span class="token function">requireNonNull</span><span class="token punctuation">(</span>remappingFunction<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">Objects</span><span class="token punctuation">.</span><span class="token function">requireNonNull</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">V</span> oldValue <span class="token operator">=</span> <span class="token function">get</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">V</span> newValue <span class="token operator">=</span> <span class="token punctuation">(</span>oldValue <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token operator">?</span> value <span class="token operator">:</span>
               remappingFunction<span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>oldValue<span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span>newValue <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token function">remove</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        <span class="token function">put</span><span class="token punctuation">(</span>key<span class="token punctuation">,</span> newValue<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> newValue<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p><code>merge()</code> 方法会先调用 <code>Objects.requireNonNull()</code> 方法判断 value 是否为空。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">T</span> <span class="token function">requireNonNull</span><span class="token punctuation">(</span><span class="token class-name">T</span> obj<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>obj <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">NullPointerException</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> obj<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h2 id="集合遍历" tabindex="-1"><a class="header-anchor" href="#集合遍历" aria-hidden="true">#</a> 集合遍历</h2><p>《阿里巴巴 Java 开发手册》的描述如下：</p><blockquote><p><strong>不要在 foreach 循环里进行元素的 <code>remove/add</code> 操作。remove 元素请使用 <code>Iterator</code> 方式，如果并发操作，需要对 <code>Iterator</code> 对象加锁。</strong></p></blockquote><p>通过反编译你会发现 foreach 语法糖底层其实还是依赖 <code>Iterator</code> 。不过， <code>remove/add</code> 操作直接调用的是集合自己的方法，而不是 <code>Iterator</code> 的 <code>remove/add</code>方法</p><p>这就导致 <code>Iterator</code> 莫名其妙地发现自己有元素被 <code>remove/add</code> ，然后，它就会抛出一个 <code>ConcurrentModificationException</code> 来提示用户发生了并发修改异常。这就是单线程状态下产生的 <strong>fail-fast 机制</strong>。</p><blockquote><p><strong>fail-fast 机制</strong> ：多个线程对 fail-fast 集合进行修改的时候，可能会抛出<code>ConcurrentModificationException</code>。 即使是单线程下也有可能会出现这种情况，上面已经提到过。</p></blockquote><p>Java8 开始，可以使用 <code>Collection#removeIf()</code>方法删除满足特定条件的元素,如</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span> list <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> i <span class="token operator">&lt;=</span> <span class="token number">10</span><span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
list<span class="token punctuation">.</span><span class="token function">removeIf</span><span class="token punctuation">(</span>filter <span class="token operator">-&gt;</span> filter <span class="token operator">%</span> <span class="token number">2</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">/* 删除list中的所有偶数 */</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>list<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">/* [1, 3, 5, 7, 9] */</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>除了上面介绍的直接使用 <code>Iterator</code> 进行遍历操作之外，你还可以：</p><ul><li>使用普通的 for 循环</li><li>使用 fail-safe 的集合类。<code>java.util</code>包下面的所有的集合类都是 fail-fast 的，而<code>java.util.concurrent</code>包下面的所有的类都是 fail-safe 的。</li><li>......</li></ul><h2 id="集合去重" tabindex="-1"><a class="header-anchor" href="#集合去重" aria-hidden="true">#</a> 集合去重</h2><p>《阿里巴巴 Java 开发手册》的描述如下：</p><blockquote><p><strong>可以利用 <code>Set</code> 元素唯一的特性，可以快速对一个集合进行去重操作，避免使用 <code>List</code> 的 <code>contains()</code> 进行遍历去重或者判断包含操作。</strong></p></blockquote><p>这里我们以 <code>HashSet</code> 和 <code>ArrayList</code> 为例说明。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// Set 去重代码示例</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">Set</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token function">removeDuplicateBySet</span><span class="token punctuation">(</span><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> data<span class="token punctuation">)</span> <span class="token punctuation">{</span>

    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token class-name">CollectionUtils</span><span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span>data<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">HashSet</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">HashSet</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>data<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">// List 去重代码示例</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token function">removeDuplicateByList</span><span class="token punctuation">(</span><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> data<span class="token punctuation">)</span> <span class="token punctuation">{</span>

    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token class-name">CollectionUtils</span><span class="token punctuation">.</span><span class="token function">isEmpty</span><span class="token punctuation">(</span>data<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token punctuation">}</span>
    <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> result <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>data<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">T</span> current <span class="token operator">:</span> data<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>result<span class="token punctuation">.</span><span class="token function">contains</span><span class="token punctuation">(</span>current<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            result<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span>current<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> result<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br></div></div><p>两者的核心差别在于 <code>contains()</code> 方法的实现。</p><p><code>HashSet</code> 的 <code>contains()</code> 方法底部依赖的 <code>HashMap</code> 的 <code>containsKey()</code> 方法，时间复杂度接近于 O（1）（没有出现哈希冲突的时候为 O（1））。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">transient</span> <span class="token class-name">HashMap</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">,</span><span class="token class-name">Object</span><span class="token punctuation">&gt;</span></span> map<span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">contains</span><span class="token punctuation">(</span><span class="token class-name">Object</span> o<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> map<span class="token punctuation">.</span><span class="token function">containsKey</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>我们有 N 个元素插入进 Set 中，那时间复杂度就接近是 O (n)。</p><p><code>ArrayList</code> 的 <code>contains()</code> 方法是通过遍历所有元素的方法来做的，时间复杂度接近是 O(n)。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">contains</span><span class="token punctuation">(</span><span class="token class-name">Object</span> o<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token function">indexOf</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span> <span class="token operator">&gt;=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token class-name">Object</span> o<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>o <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> size<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>elementData<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token operator">==</span><span class="token keyword">null</span><span class="token punctuation">)</span>
                <span class="token keyword">return</span> i<span class="token punctuation">;</span>
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> size<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>o<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>elementData<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
                <span class="token keyword">return</span> i<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>我们的 <code>List</code> 有 N 个元素，那时间复杂度就接近是 O (n^2)。</p><h2 id="集合转数组" tabindex="-1"><a class="header-anchor" href="#集合转数组" aria-hidden="true">#</a> 集合转数组</h2><p>《阿里巴巴 Java 开发手册》的描述如下：</p><blockquote><p><strong>使用集合转数组的方法，必须使用集合的 <code>toArray(T[] array)</code>，传入的是类型完全一致、长度为 0 的空数组。</strong></p></blockquote><p><code>toArray(T[] array)</code> 方法的参数是一个泛型数组，如果 <code>toArray</code> 方法中没有传递任何参数的话返回的是 <code>Object</code>类 型数组。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">String</span> <span class="token punctuation">[</span><span class="token punctuation">]</span> s<span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">{</span>
    <span class="token string">&quot;dog&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;lazy&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;a&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;over&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;jumps&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;fox&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;brown&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;quick&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;A&quot;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> list <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">asList</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Collections</span><span class="token punctuation">.</span><span class="token function">reverse</span><span class="token punctuation">(</span>list<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//没有指定类型的话会报错</span>
s<span class="token operator">=</span>list<span class="token punctuation">.</span><span class="token function">toArray</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>由于 JVM 优化，<code>new String[0]</code>作为<code>Collection.toArray()</code>方法的参数现在使用更好，<code>new String[0]</code>就是起一个模板的作用，指定了返回数组的类型，0 是为了节省空间，因为它只是为了说明返回的类型。详见：<a href="https://shipilev.net/blog/2016/arrays-wisdom-ancients/" target="_blank" rel="noopener noreferrer">https://shipilev.net/blog/2016/arrays-wisdom-ancients/<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a></p><h2 id="数组转集合" tabindex="-1"><a class="header-anchor" href="#数组转集合" aria-hidden="true">#</a> 数组转集合</h2><p>《阿里巴巴 Java 开发手册》的描述如下：</p><blockquote><p><strong>使用工具类 <code>Arrays.asList()</code> 把数组转换成集合时，不能使用其修改集合相关的方法， 它的 <code>add/remove/clear</code> 方法会抛出 <code>UnsupportedOperationException</code> 异常。</strong></p></blockquote><p>我在之前的一个项目中就遇到一个类似的坑。</p><p><code>Arrays.asList()</code>在平时开发中还是比较常见的，我们可以使用它将一个数组转换为一个 <code>List</code> 集合。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> myArray <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token string">&quot;Apple&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;Banana&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;Orange&quot;</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> myList <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">asList</span><span class="token punctuation">(</span>myArray<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//上面两个语句等价于下面一条语句</span>
<span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> myList <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">asList</span><span class="token punctuation">(</span><span class="token string">&quot;Apple&quot;</span><span class="token punctuation">,</span><span class="token string">&quot;Banana&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;Orange&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>JDK 源码对于这个方法的说明：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token doc-comment comment">/**
  *返回由指定数组支持的固定大小的列表。此方法作为基于数组和基于集合的API之间的桥梁，
  * 与 Collection.toArray()结合使用。返回的List是可序列化并实现RandomAccess接口。
  */</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token function">asList</span><span class="token punctuation">(</span><span class="token class-name">T</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> a<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>下面我们来总结一下使用注意事项。</p><p><strong>1、<code>Arrays.asList()</code>是泛型方法，传递的数组必须是对象数组，而不是基本类型。</strong></p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">int</span><span class="token punctuation">[</span><span class="token punctuation">]</span> myArray <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token class-name">List</span> myList <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">asList</span><span class="token punctuation">(</span>myArray<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>myList<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//1</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>myList<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//数组地址值</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>myList<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//报错：ArrayIndexOutOfBoundsException</span>
<span class="token keyword">int</span><span class="token punctuation">[</span><span class="token punctuation">]</span> array <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> myList<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>array<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//1</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>当传入一个原生数据类型数组时，<code>Arrays.asList()</code> 的真正得到的参数就不是数组中的元素，而是数组对象本身！此时 <code>List</code> 的唯一元素就是这个数组，这也就解释了上面的代码。</p><p>我们使用包装类型数组就可以解决这个问题。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">Integer</span><span class="token punctuation">[</span><span class="token punctuation">]</span> myArray <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br></div></div><p><strong>2、使用集合的修改方法: <code>add()</code>、<code>remove()</code>、<code>clear()</code>会抛出异常。</strong></p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">List</span> myList <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">asList</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
myList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//运行时报错：UnsupportedOperationException</span>
myList<span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//运行时报错：UnsupportedOperationException</span>
myList<span class="token punctuation">.</span><span class="token function">clear</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//运行时报错：UnsupportedOperationException</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><code>Arrays.asList()</code> 方法返回的并不是 <code>java.util.ArrayList</code> ，而是 <code>java.util.Arrays</code> 的一个内部类,这个内部类并没有实现集合的修改方法或者说并没有重写这些方法。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">List</span> myList <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">asList</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>myList<span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//class java.util.Arrays$ArrayList</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>下图是 <code>java.util.Arrays$ArrayList</code> 的简易源码，我们可以看到这个类重写的方法有哪些。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code>  <span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">extends</span> <span class="token class-name">AbstractList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span>
        <span class="token keyword">implements</span> <span class="token class-name">RandomAccess</span><span class="token punctuation">,</span> <span class="token class-name"><span class="token namespace">java<span class="token punctuation">.</span>io<span class="token punctuation">.</span></span>Serializable</span>
    <span class="token punctuation">{</span>
        <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>

        <span class="token annotation punctuation">@Override</span>
        <span class="token keyword">public</span> <span class="token class-name">E</span> <span class="token function">get</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>
          <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
        <span class="token punctuation">}</span>

        <span class="token annotation punctuation">@Override</span>
        <span class="token keyword">public</span> <span class="token class-name">E</span> <span class="token function">set</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">,</span> <span class="token class-name">E</span> element<span class="token punctuation">)</span> <span class="token punctuation">{</span>
          <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
        <span class="token punctuation">}</span>

        <span class="token annotation punctuation">@Override</span>
        <span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">indexOf</span><span class="token punctuation">(</span><span class="token class-name">Object</span> o<span class="token punctuation">)</span> <span class="token punctuation">{</span>
          <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
        <span class="token punctuation">}</span>

        <span class="token annotation punctuation">@Override</span>
        <span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">contains</span><span class="token punctuation">(</span><span class="token class-name">Object</span> o<span class="token punctuation">)</span> <span class="token punctuation">{</span>
           <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
        <span class="token punctuation">}</span>

        <span class="token annotation punctuation">@Override</span>
        <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">forEach</span><span class="token punctuation">(</span><span class="token class-name">Consumer</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span> <span class="token keyword">super</span> <span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> action<span class="token punctuation">)</span> <span class="token punctuation">{</span>
          <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
        <span class="token punctuation">}</span>

        <span class="token annotation punctuation">@Override</span>
        <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">replaceAll</span><span class="token punctuation">(</span><span class="token class-name">UnaryOperator</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> operator<span class="token punctuation">)</span> <span class="token punctuation">{</span>
          <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
        <span class="token punctuation">}</span>

        <span class="token annotation punctuation">@Override</span>
        <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">sort</span><span class="token punctuation">(</span><span class="token class-name">Comparator</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span> <span class="token keyword">super</span> <span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> c<span class="token punctuation">)</span> <span class="token punctuation">{</span>
          <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br><span class="line-number">36</span><br><span class="line-number">37</span><br><span class="line-number">38</span><br><span class="line-number">39</span><br><span class="line-number">40</span><br></div></div><p>我们再看一下<code>java.util.AbstractList</code>的 <code>add/remove/clear</code> 方法就知道为什么会抛出 <code>UnsupportedOperationException</code> 了。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">E</span> <span class="token function">remove</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">UnsupportedOperationException</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token class-name">E</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">add</span><span class="token punctuation">(</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> e<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">,</span> <span class="token class-name">E</span> element<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">UnsupportedOperationException</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">clear</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">removeRange</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">removeRange</span><span class="token punctuation">(</span><span class="token keyword">int</span> fromIndex<span class="token punctuation">,</span> <span class="token keyword">int</span> toIndex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">ListIterator</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> it <span class="token operator">=</span> <span class="token function">listIterator</span><span class="token punctuation">(</span>fromIndex<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">,</span> n<span class="token operator">=</span>toIndex<span class="token operator">-</span>fromIndex<span class="token punctuation">;</span> i<span class="token operator">&lt;</span>n<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        it<span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><p><strong>那我们如何正确的将数组转换为 <code>ArrayList</code> ?</strong></p><p>1、手动实现工具类</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">//JDK1.5+</span>
<span class="token keyword">static</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token function">arrayToList</span><span class="token punctuation">(</span><span class="token keyword">final</span> <span class="token class-name">T</span><span class="token punctuation">[</span><span class="token punctuation">]</span> array<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">final</span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> l <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>array<span class="token punctuation">.</span>length<span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">final</span> <span class="token class-name">T</span> s <span class="token operator">:</span> array<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    l<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> l<span class="token punctuation">;</span>
<span class="token punctuation">}</span>


<span class="token class-name">Integer</span> <span class="token punctuation">[</span><span class="token punctuation">]</span> myArray <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token function">arrayToList</span><span class="token punctuation">(</span>myArray<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//class java.util.ArrayList</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><p>2、最简便的方法</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">List</span> list <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">asList</span><span class="token punctuation">(</span><span class="token string">&quot;a&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;b&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;c&quot;</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br></div></div><p>3、使用 Java8 的 <code>Stream</code>(推荐)</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">Integer</span> <span class="token punctuation">[</span><span class="token punctuation">]</span> myArray <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token class-name">List</span> myList <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span>myArray<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span><span class="token class-name">Collectors</span><span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//基本类型也可以实现转换（依赖boxed的装箱操作）</span>
<span class="token keyword">int</span> <span class="token punctuation">[</span><span class="token punctuation">]</span> myArray2 <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token class-name">List</span> myList <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">stream</span><span class="token punctuation">(</span>myArray2<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">boxed</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span><span class="token class-name">Collectors</span><span class="token punctuation">.</span><span class="token function">toList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>4、使用 Guava</p><p>对于不可变集合，你可以使用<a href="https://github.com/google/guava/blob/master/guava/src/com/google/common/collect/ImmutableList.java" target="_blank" rel="noopener noreferrer"><code>ImmutableList</code><span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>类及其<a href="https://github.com/google/guava/blob/master/guava/src/com/google/common/collect/ImmutableList.java#L101" target="_blank" rel="noopener noreferrer"><code>of()</code><span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>与<a href="https://github.com/google/guava/blob/master/guava/src/com/google/common/collect/ImmutableList.java#L225" target="_blank" rel="noopener noreferrer"><code>copyOf()</code><span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>工厂方法：（参数不能为空）</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> il <span class="token operator">=</span> <span class="token class-name">ImmutableList</span><span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token string">&quot;string&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;elements&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">// from varargs</span>
<span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> il <span class="token operator">=</span> <span class="token class-name">ImmutableList</span><span class="token punctuation">.</span><span class="token function">copyOf</span><span class="token punctuation">(</span>aStringArray<span class="token punctuation">)</span><span class="token punctuation">;</span>      <span class="token comment">// from array</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>对于可变集合，你可以使用<a href="https://github.com/google/guava/blob/master/guava/src/com/google/common/collect/Lists.java" target="_blank" rel="noopener noreferrer"><code>Lists</code><span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>类及其<a href="https://github.com/google/guava/blob/master/guava/src/com/google/common/collect/Lists.java#L87" target="_blank" rel="noopener noreferrer"><code>newArrayList()</code><span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>工厂方法：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> l1 <span class="token operator">=</span> <span class="token class-name">Lists</span><span class="token punctuation">.</span><span class="token function">newArrayList</span><span class="token punctuation">(</span>anotherListOrCollection<span class="token punctuation">)</span><span class="token punctuation">;</span>    <span class="token comment">// from collection</span>
<span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> l2 <span class="token operator">=</span> <span class="token class-name">Lists</span><span class="token punctuation">.</span><span class="token function">newArrayList</span><span class="token punctuation">(</span>aStringArray<span class="token punctuation">)</span><span class="token punctuation">;</span>               <span class="token comment">// from array</span>
<span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> l3 <span class="token operator">=</span> <span class="token class-name">Lists</span><span class="token punctuation">.</span><span class="token function">newArrayList</span><span class="token punctuation">(</span><span class="token string">&quot;or&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;string&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;elements&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// from varargs</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>5、使用 Apache Commons Collections</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> list <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">CollectionUtils</span><span class="token punctuation">.</span><span class="token function">addAll</span><span class="token punctuation">(</span>list<span class="token punctuation">,</span> str<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>6、 使用 Java9 的 <code>List.of()</code>方法</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">Integer</span><span class="token punctuation">[</span><span class="token punctuation">]</span> array <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span> list <span class="token operator">=</span> <span class="token class-name">List</span><span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span>array<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre><div class="line-numbers" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><!--]--></div><!----><footer class="page-meta"><div class="meta-item edit-link"><a href="https://github.com/Snailclimb/JavaGuide/edit/main/docs/java/collection/java-collection-precautions-for-use.md" rel="noopener noreferrer" target="_blank" arialabel="编辑此页" class="nav-link label"><!--[--><svg xmlns="http://www.w3.org/2000/svg" class="icon edit-icon" viewbox="0 0 1024 1024" arialabelledby="edit"><title id="edit" lang="en">edit icon</title><g fill="currentColor"><path d="M430.818 653.65a60.46 60.46 0 0 1-50.96-93.281l71.69-114.012 7.773-10.365L816.038 80.138A60.46 60.46 0 0 1 859.225 62a60.46 60.46 0 0 1 43.186 18.138l43.186 43.186a60.46 60.46 0 0 1 0 86.373L588.879 565.55l-8.637 8.637-117.466 68.234a60.46 60.46 0 0 1-31.958 11.229z"></path><path d="M728.802 962H252.891A190.883 190.883 0 0 1 62.008 771.98V296.934a190.883 190.883 0 0 1 190.883-192.61h267.754a60.46 60.46 0 0 1 0 120.92H252.891a69.962 69.962 0 0 0-69.098 69.099V771.98a69.962 69.962 0 0 0 69.098 69.098h475.911A69.962 69.962 0 0 0 797.9 771.98V503.363a60.46 60.46 0 1 1 120.922 0V771.98A190.883 190.883 0 0 1 728.802 962z"></path></g></svg><!--]-->编辑此页<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span><!----></a></div><div class="meta-item update-time"><span class="label">上次编辑于: </span><span class="info">2022/3/3 09:14:56</span></div><div class="meta-item contributors"><span class="label">贡献者: </span><!--[--><!--[--><span class="contributor" title="email: koushuangbwcx@163.com">guide</span><!--]--><!--]--></div></footer><nav class="page-nav"><a href="/java/collection/java-collection-questions-02.html" class="nav-link prev" arialabel="Java集合常见知识点&amp;面试题总结(下)"><div class="hint"><span class="arrow left"></span>上一页</div><div class="link"><!---->Java集合常见知识点&amp;面试题总结(下)</div></a><!----></nav><!----><!----></main><!--]--><footer class="footer-wrapper"><div class="footer"><a href="https://beian.miit.gov.cn/" target="_blank">鄂ICP备2020015769号-1</a></div><div class="copyright">Copyright © 2022 Guide</div></footer></div><!--]--><!----><!--]--></div>
    <script type="module" src="/assets/app.93341f6d.js" defer></script>
  </body>
</html>
